If you are trying to replace a face, then I think you will be out of luck with Motion.
Getting an object to match the movement of a face should be possible, if the face doesn't move around too much.
You can use Match move and track two points on the face, that lets you match movement on the X and Y axes, rotation on the Z axis and scale.
I was looking to do more of a mask on the face. The project is of a bag of cheese and the cheese would have a face that is the talent video taped and I wanted to lay the cheese bag over the talents face and have it appear as the face was pushing out of the bag. I thought it might be to much so I might have to come up with something else.I'm on a tight deadline. Thanks for the response.